This invention relates to a method and apparatus for hard machining a hardened workpiece, and more particularly to such methods and apparatus in which the cutting tool is moved to bring a fresh, unworn portion of a cutting edge into cutting engagement with the workpiece.
There is growing interest in a machining operation known as xe2x80x9chard turningxe2x80x9d or xe2x80x9chard machining.xe2x80x9d This involves removing material from a workpiece in a machining operation with the workpiece in a hardened state. In the past, it has been customary to produce parts, such as ball and roller bearings, gears, cams, etc., that must be hardened to decrease wear, by the following sequence of steps:
rough machine a part in its soft state,
heat treat the part, and
rough and finish grind the part to provide desired accuracy and surface finish.
By contrast, in hard machining, the hardened part is machined to produce a part in a single operation instead of the more costly sequence indicated above.
Hard machining has become an option with the appearance of improved tool materials such as cubic boron nitride or polycrystalline cubic boron nitride (hereinafter xe2x80x9cCBNxe2x80x9d or xe2x80x9cPCBN,xe2x80x9d respectively) and ceramics. The CBN or PCBN is very expensive (comparable to diamond in this respect), while the latter, hard-cutting ceramics, have a much shorter tool life, but a much lower cost. It would be possible to use the less expensive ceramic tool material, a superior grade of carbide or other low cost tool material capable of hard machining, provided the nonproductive tool changing time could be reduced and the tool material could be used more efficiently. As used herein, then, a hard machining cutting tool is one having a cutting edge of one of the aforesaid materials capable of hard machining, to wit CBN, PCBN, hard-cutting, ceramics, superior grade carbide or other tool material capable of hard machining.
An improved method and apparatus for combining a comparatively coarse, roughing cut at a high removal rate (in a location of coarse cutting) and a finishing cut at a low removal rate (in a location of finishing cutting) are employed that are useful in hard turning operations. The new method and apparatus make it possible to use a much less expensive tool material such as ceramic in place of PCBN that is now used in hard turning operations. This is accomplished by offsetting the lower tool life of a lower cost tool relative to that of a PCBN tool by providing the tool with an extended cutting edge extending along a path of translation and moving the tool to move the cutting edge along the path of translation so that a fresh cutting edge portion is brought rapidly into cutting position. In particular, in a preferred embodiment of the invention, a fresh cutting edge portion is moved into position to replace the portion of a cutting edge being used for the finishing cut, which finishing portion of the cutting edge is less than the entire portion in engagement with the workpiece.
In a specific preferred embodiment, the method and apparatus of the invention achieves the above objectives by:
using a large diameter cylindrical tool of ceramic having a large number of new cutting edges (or cutting portions of the continuous circular cutting edge) along the periphery of the face of a single cylindrical tool, and reducing the nonproductive downtime to change tools to essentially zero by merely rotating the cylindrical tool through a small arc when a new cutting edge is required.
The substitution of a ceramic tool material for CBN or PCBN has an important thermal advantage in the combined roughing/finishing cut employed in hard turning. CBN has a much higher thermal diffusivity than does a ceramic. While this is advantageous relative to tool wear in conventional machining applications, this is not the case for the type of cut employed in hard turning as shown in FIG. 1, where is the depth of cut and f is the feed per revolution (fpr). In this case, a chip region of large thickness (tr) responsible for most of the removal (and hence temperature rise) is contiguous with the finishing region of the chip having low thickness (tf) (and hence low removal rate and low temperature rise). The finish of the surface produced is influenced primarily by the thickness of the chip tf, while the bulk of the material is removed in the region where the chip thickness is tr. The bulk of the heat generated and hence most of the tool wear will be in the tr region. Relative to surface finish, tool wear in the tf (finishing cut) region is more important than that in the tr (comparatively coarse cut) roughing region.
Surface finish deterioration, the rate of which increases with temperature, determines the useable tool life in hard turning. It is therefore important that the higher temperature in the roughing region (region of large tr) not flow into the finishing region (region of low tf) to reduce tool life. The poorer thermal properties of ceramic relative to CBN is advantageous in this particular case. Also, a larger tool radius can mean a large arc of tool engagement and a longer heat travel distance from the hottest portion of the roughing region to the comparatively cooler finishing region.